The present invention relates to focusing position detection apparatus for speedily detecting a focusing position using a single photoelectric conversion array.
Conventionally, in ordinary range finder cooperative type cameras, a focusing position for lenses in the camera is detected by bringing double images in line with each other. In single-lens reflex cameras, photoelectric conversion elements, placed on an image formation surface or on an equivalent surface thereto, are employed. In the latter case, it is impossible to tell whether or not a focusing position is obtained only by an initial measurement. Referring to FIG. 1, supposing that a film surface FL is an image formation surface of a photographing lens LL and that a photoelectric conversion element PS is positioned in contact with the film surface FL and an object is focused at a distance La (which is not a minimum photographing distance, but a definite distance), which corresponds to the distance Fa between the film surface FL and the lens LL. As shown in FIG. 2, when the distance of lens movement (for instance, by setting the film side as positive direction) is plotted as abscissa and the output of a photoelectric conversion element PS as ordinate, the output of the photoelectric conversion element PS is Sp at an initial position Fp of the lens LL where the image is not focused. When an output Sq is obtained by moving the lens LL slightly, if Sq is greater than Sp, it is judged that such movement of the lens LL is correct and the lens LL is then further moved to Fr. At that time, an output Sr is obtained. With a further movement of the lens LL to Fa, an output Sa is obtained and with a still further movement of the lens LL to Ft, an output St is obtained. While moving the lens LL in the positive direction, a position Ft is obtained where the output of the photoelectric conversion element PS is decreased so that St becomes smaller than Sa, whereby it is judged that a focused position is Fa which is immediately before the position Ft. This procedure takes a lot of time before finding the focused position since the above-mentioned step has to be taken a number of times. If the initial position is at Fr with the output Sr obtained and the lens LL is then moved to the position Ft, slightly passing over the focused position, and the output St is obtained, Sr is greater than St and accordingly it is judged that the focused position is not in the direction of Ft from the viewpoint of Fr. As a result, the focused position has to be searched from the direction Fq. This situation is apt to occur when Fr is close to Fa. Furthermore, when Sq and St have an equal value, it cannot be decided in which direction the lens LL should be moved and, if the lens LL happens to be in the focusing position, the lens LL has to be moved from that position before concluding that the lens LL has already been in the focusing position. In any case, a complicated operation is required.
In order to eliminate such a complicated operation, a construction as shown in FIG. 3 is proposed, in which one additional photoelectric conversion element is used. Namely, a second photoelectric conversion element PS2 is positioned with a space d, in the direction of the optical axis, from a first photoelectric conversion element PS1. These two elements PS1 and PS2 produce their respective outputs with respect to the incident light from the same object. With respect to the output S1 of the element PS1 and the output S2 of the element PS2, the outputs at the two positions which are spaced slightly from each other are compared in their magnitude as done in the case of FIG. 1, so that the direction of the lens movement can be determined speedily. However, in this apparatus, two photoelectric conversion elements PS1 and PS2 have to be disposed and their respective outputs S1 and S2 have to be compared with each other. As a result, the apparatus becomes complex in the mechanism and oversized. Therefore, it is not suitable for small size cameras.